Hydrogen storage alloys based on misch metal (Mm) are commercially used as negative electrode materials for the nickel-hydride battery. In general, cobalt is contained in the alloys to guarantee the long cycle life of the negative electrode. Consequently, cobalt is an essential element in the hydrogen storage alloy. In order to elucidate the function of cobalt in the charge and discharge processes, the electrochemical feature of the negative electrode as well as the diffusion behavior of hydrogen in the alloy were systematically examined for the MmNi(4.0-x)Mn(0.75)Al(0.25)Co(x) (0 less than or equal to x less than or equal to 0.6) alloys as a function of the cobalt content. The results of electrochemical measurements are interpreted from the viewpoint of the changes in unit cell volume of the alloys that strongly influences the stability of the hydride.